The invention describes a high power laser diode with potential use in ophthalmology, surgery, printing, magneto-optical data storage.
There are already known high power laser diodes made of symmetrical structures with separate confinement in which the light is emitted in a very narrow active region and is guided by a larger wave guide. These diodes have the following disadvantages:
they are optimized for low threshold current and as a consequence they have an as high as possible confinement factor (the ratio of the power density through the active region to the total power density) and that induces the catastrophical degradation of the mirror at relative small power densities and allows the optimal operation only for short devices, due to the high value of the modal gain;
they have reduced operational safety due to the low value of the active region thickness;
they are not optimized for maximum power densities, relative to the emitting stripe width, optimization for which two conditions have to be fulfilled: a low value of the confinement factor ( less than 0.01) together with a high value of the active region thickness ( greater than 10 nm).
There are already known high power laser diodes structures with low confinement factor, in the range 0.0015 and 0.00015 (patent RO 102871, PCT Application RO 91/0002), consisting of a main region of the wave guide (2-5 xcexcm thickness) and of an active region, lateral to the main region and separated from it by an intermediate region. The radiation propagates mainly in the main region, which is very large, and the cut-off of the high-order modes is realized by a very small step of the refractive index value between the confinement region and the main region. They have the following disadvantages:
the imposed step of the refractive index between the main region and the confinement region is very small and very difficult to be realized from the technological point of view,
the passive region confinement factor value is almost equal to unity and its band gap is very close to the energy of the propagating photons, conditions which together can enhance two photon absorption.
The problem solved by this invention is the designing of a laser diode structure made of multiple layers which operates with a low value of the confinement factor less, than 0.015, and which has the thickness of the active region greater than 0.01 xcexcm and small values for the confinement factor of the small energy gap layers, others than the active region.
The high power laser diode structures, corresponding to this invention, eliminate the disadvantages of the other known solutions since they are comprised of an n-type confinement region; a p-type confinement region which has the lowest value of the refractive index in the structure; a passive region situated between the two confinement regions which represents the main part of the wave guide and whose refractive index may be lowered stepwise or continuously relative to the refractive index of the n-type confinement region, this decrease being greater at (towards) the limit of separation between the passive region and the p-type confinement region; a thin active region situated asymmetrically in the passive region and closer to the p-type confinement region and whose refractive index is much greater than the refractive index of the passive region; a thin balance region situated at the limit between the passive region and the n-type confinement region, which balance the action of the active region and whose refractive index is much higher than the refractive index of its neighbor regions, i.e. the passive region and the n-type confinement region; the structure being such that the refractive index profile has two marked bumps, one corresponding to the active region and the other corresponding to the balance region, both bumps having the magnitude less than xcex/4. due to the very small thickness of these two regions; and such that the energy band gap of the balance region is much greater than the energy band gap of the active region to avoid the absorption in the balance region of the radiation emitted in the active region; and since, by the combined action of two factors, the prevalent attraction of the field by the balance region as compared with the attraction of the active region and the repelling toward the n-type confinement region of the field due to the refractive index step at the margin between the p-type confinement region and the passive region, the named laser diode structures assures a field distribution with its maximum situated in the balance region, outside the active region bump, and a reduced value for the confinement factor of the active region by repelling the field distribution maximum from the active region, together with a reduced value of the confinement factor of the balance region due to its low thickness.
The high power laser diodes according to the invention present the following advantages:
they have a reduced confinement factor, lower than 0.015, although the active region is rather thick, thicker than 10 nm;
they operate in the fundamental transversal mode;
they have low confinement factor for low band gap regions others than the active region;
they allow the fabrication of stripes with low modal gain.